(a) Field of the Invention
The present invention relates to the technique for the distillation of air in the production of gaseous oxygen under variable operating conditions, by means of an arrangement provided with a double column. It mainly concerns a process of the type in which, when the demand of gaseous oxygen is lower than an average value, liquid oxygen is allowed to pass from the low pressure column of the double column into a first container for storing liquid oxygen and liquid nitrogen originating from a second storage container for liquid nitrogen is sent to the double column, whilst when the demand of gaseous oxygen is higher than the average value, liquid oxygen withdrawn from the first storage container is introduced into the low pressure column and a corresponding quantity of liquid nitrogen is condensed simultaneously, said liquid nitrogen being sent to the second storage container.
(b) Description of Prior Art
The process of this type are generally called "aternating". A distinction is made between two different operations of the arrangement:
(a) when the demand of gaseous oxygen is high, the excess of gaseous oxygen which is withdrawn from the low pressure column is compensated, on a point of view of balance of material by an injection, into this column, of a corresponding quantity of liquid oxygen which originates from the first storage container. To equilibrate the refrigerating balance of the double column, a corresponding quantity of liquid nitrogen is sent from the mean pressure column to the second storage container. For this reason, this operation is generally called "operation NL", and this is the designation which will be used hereinafter.
(b) When the demand of gaseous oxygen is low, the lack of oxygen with respect to the nominal operation is sent in liquid form from the low pressure column to the first storage container (from which the expression "operation OL" which will often be used). The refrigerating balance is obtained by injection into the low pressure column of a corresponding quantity of liquid nitrogen which originates from the second storage container.
When the arrangement includes a column for the production of impure argon coupled to the low pressure column, this alternating principle is not without influence on the yield of extraction of argon. As a matter of fact:
(a) in operation NL, it is easy to realize that the vapor/liquid ratio decreases in the lower portion of the low pressure column, which is detrimental to the oxygen/argon separation that takes place in this zone, and therefore to the yield of extraction of argon.
There is obviously another phenomenon which acts in reversed direction: the vaporisation of additional liquid oxygen condenses a higher quantity of nitrogen at the top of the mean pressure column since the heat of vaporisation of nitrogen is about 20% lower than that of oxygen. There is therefore a deficit of gaseous products which is sent to the cold end of the heat exchanger line of the arrangement, which closes the heat exchange diagram towards the hot end and results in an increase of the suction temperature of the turbine which is generally used to keep the arrangement cold. This turbine has an increased specific refrigerating power, which enables to decrease the flow produced by the turbine. However, this favorable phenomenon is truly less important than the unfavorable phenomenon mentioned above consisting of a reduction of the vapor/liquid ratio. In all, the operation NL is thus unfavorable to the yield of extraction of argon.
(b) In operation OL, the reverse takes place, for similar reasons: on the other hand, the vapor/liquid ratio increases at the bottom of the low pressure column, and on the other hand, the suction temperature of the turbine decreases, which requires to increase the flow of gas produced by the turbine. In all, this operation OL promotes the yield of extraction of argon.
(c) Unfortunately, the increase in the yield of extraction of argon obtained in OL operation is less important than the loss of yield of extraction of argon which takes place in operation NL, since the yield corresponding to the nominal operation is generally high and already near an asymptote. The result is that if, and this will be the basic hypothesis, the nominal operation corresponds to the mean production of oxygen in the arrangement, then the alternating process lowers the mean yield during the extraction of argon.